Microphone with low frequency filter



March 28, 1967 c. E- GRINSTEAD 3,311,703

MICROPHONE WITH LOW FREQUENCY FILTER Filed May 21, 1963 INVENTOR.

BY v a H TI'ORNEKS.

United States Patent 3,311,703 MICROPHONE WITH LOW FREQUENCY FILTER Carl E. Grinstead, 1600 S. Baldwin, Arcadia, Calif. 91006 Filed May 21, 1963, Ser. No. 282,050 13 Claims. (Cl. 179-1) This invention relates to sound detection devices and more particularly, to a microphone for the detection of sounds emanating from the heart.

An object of this invention is to provide a microphone and a readout means for detecting, measuring and reading out frequencies emitted by the human body, such as by its heart, which will not be obscured by concomitant lowfrequency effects such as results from breathing, gradual body movements, and the like.

An additional object of the invention is to provide means for accomplishing the foregoing in a device which remains tuned for optimum effect in the presence of said low-frequency effects, Which has means for varying the sensitivity of the microphone, and which is designed to make a close coupling with the chest of the body for optimum transmission without upsetting relationships between internal electrical components of the microphone.

The invention will be fully understood from the following detailed description and accompanying drawings in which:

FIG. 1 is a cross-sectional view of the microphone and taken at line 1--1 of FIG. 2, and line 1+1 of FIG. 3;

FIG. 2 is an end view taken at line 2-2 of FIG. 1; and

FIG. 3 is an end view of the microphone taken at line 3--3 of FIG. 1.

. The drawings show a microphone comprising a coil housing 11, a body 12, and a housing adapter 13 to connect the coil housing 11 to the body 12 along a longitudinal axis 14. A co-axial receptacle 15, such as Microdot No. 13-01, Series S50 manufactured by Microbearing against a side of the insulated disc 26 and end of the shank of the rivet opposite the head of the rivet is swaged outwardly to rigidly attach the contact spring retainer to the disc 26. p

In assembling the various components into the coil housing, wire 24 is soldered to stem 21 and receptacle 15 is then threaded into hole 16 of the coil housing. Insulator 22 is then inserted into the cavity 17 of the housing and wire 24 is then attached to the primary winding 19 on coil spool 18. Spool 18 is then inserted into the housing and ferrite sleeve 25 and copper sleeve 27 are inserted into their respective positions. The unconnected end of primary winding 19 and an end of the secondary winding are soldered at point 33 to the inside Wall of copper sleeve 27 which has the same potential as the coil housing and is at ground potential When the dot Incorporated, South Pasadena, California, is threaded into a hole 16 in-an end of coil housing 11.

The coil housing 11 has an inner cavity 17 and a cylindrical coil spool 18 disposed therein. Coil spool 18 is made of a ferrite material such as powdered iron, and has a primary winding 19 and a secondary winding 20 wound around the spool as two windings of a transformer. The co-axial receptacle 15 is spaced from the spool by an insulator 22 and has an inner stem 21 insulated from said housing. Insulator 22 has an inner cavity 23 to allow a wire 24 to pass from the stem 21 to the primary winding 19. A tubular coil sleeve 25 extends around the periphery of the coil and is made of a ferrite material such as powderediron. The purposeof the sleeve 25 is to complete the magnetic circuit for the windings on spool 18 and to shield the windings from any extraneous magnetic fields which would hinder operation of the device. A disc 26 is spaced from the coil spool at the end of the coil housing opposite the co-axial connector 15 and has an outer periphery defined by the inner wall of cavity 17 of coil housing 11. copper sleeve 27 is disposed in the coil housing 11 and has a length defined by the spacing between insulator 22 and disc 26 and a wall thickness defined by the outer periphery of coil sleeve 25 and the inner wall of cavity 17 of the coil housing. The disc is disposed within the cavity 17 and a crimp 28 is formed on the end of housing 11 to hold the disc assembled therewith.

The disc 26 is made of an insulating material and has a central hole 29. A spring 30, being conical in shape, is attached by its base to the insulator disc 26 at hole 29 by a contact spring retainer 31 and a tubular contact spring retainer rivet 32. The shank of the retainer rivet 32 passes through hole 29 with the head of the rivet A tubular 7 microphone is in operation. The unconnected end of the secondary Winding 20 is soldered to contact spring retainer rivet 32 which is electrically connected to contact spring 30 through the contact spring retainer 31. A capacitor 34 is connected between rivet 32 and the copper sleeve 27. Disc 26 with the spring attached is inserted within the cavity to bear against the end of copper sleeve 27 and the end of the housing adjacent disc 26 is swaged inwardly to securely hold the disc against sleeve 27. A bonding material with good electrical insulation characteristics may then be inserted into the space between the spool 18 and the disc 26 to insure that there will be no shorting of wires in the space.

Housing adapter 13 is a tubular shaped member attached to coil housing 11 and extending longitudinally away from the end of the coil housing opposite eo-axial connector 15. At the end of housing adapter 13 is attached an electrode 35 insulated from housing adapter 13 by an insulating disc 36. The electrode has a shank 37 passing through the disc and making electrical contact with the contact spring 30. The electrode is secured to the insulating disc by a nut threaded onto the shank. A layer of bonding material 38 is applied to the insulating disc on the inside of the housing adapter and has a thickness sufiicient to secure the insulated disc and electrode rigidly to the housing adapter 13. Coil housing 11 and housing adapter 13 are threaded so that the housing adapter may be rigidly attached to the coil housing. The two windings of the coil are mutually inductive and their assembly acts as a current transformer wherein the secondary has a greater number of turns than the primary. For example, the secondary winding has three hundred turns and the primary winding has eight turns. A low value temperature-sensitivecapacitor 34 such as 8.5 mrnf. is connected from the brass rivet 32 to the cooper sleeve 27 for the purpose of maintaining impedance stability of the resonant circuit should the microphone be exposed to changes in temperature during operation. The capacitors capacitance decreases with increasing temperature to olfset the concomitant increase in inductance of the coil.

The body 12 is a tubular shaped member with a centrally located hole 39 at one end to accept the end of the housing adapter 13 opposite the coil housing 11. The body 12 has a frusto-conical cavity 40 to enclose the electrode 35. Two holes spaced diametrically opposite each other on the body 12 are tapped to accept a lock screw 41 and a guide screw 42. The holes are disposed such that the screws 41 and 42 are brought to bear against two chamfered keyways, 43 and 43a respectively, on housing adapter 13. The keyways 43 and 43a extend longitudinally along the outer wall of the housing adapter 13 such that when the locking screw 41 is loosened, the housing adapter and coil housing 11 may be moved independently of the body 12.

An adjusting barrel 44 is threaded onto-the housing adapter 13 and disposed between the body 12 and the coil housing 11. The adjusting barrel 44 is rotatable about the housing adapter and is guided by a locking ring 45 rigidly attached to body 12 at an end thereof. For assembly and removal of body 12 from the adjusting barrel the locking ring 45 comprises two semicircular rings 46 and 47 attached to the end of the body 12 as shown in FIG. 2. Ring 45 has an inner periphery defined by the bottom of an annular groove 48 in the outer wall of the adjusting barrel 44. Adjusting barrel 44 is rotatable about longitudinal axis 14 and body 12, and is held in a fixed longitudinal position to body 12 by the locking ring. A flange 49 extends outwardly from the wall of the adjusting barrel and is disposed adjacent the locking ring 45. The flange 49 has a notch 50 to allow the removal of screws such as 51 which attach the locking ring 45 to the body 12. v

A diaphragm 52 is spaced from the electrode 35 and is attached by a clamping ring 53 to the body 12 by the use of screws such as 54 shown in FIG. 3. The diaphragm 52 is metallized with a layer 55 of an electrically conductive material such as aluminum. The diaphragm is made of a thin flexible material such as Mylar. The diaphragm is approximately 0.0005 thick and the metallized layer 55 is caused to be at ground potential by screws 54. The metallized diaphragm acts as one plate of an air gap capacitor, with electrode 35 acting as the other plate. The electrode has a face 56 disposed adjacent metallized diaphragm 52 and substantially parallel to the diaphragm. The metal layer 55 is on the opposite side of the diaphragm from the electrode, thereby adding the capacitance of the diaphragm to the system. This is advantageous in that the electrode 35 and metallized layer 55 will never come into contact with each other.

The adjusting barrel is held captive to the body 12 by ring 45 when the barrel is rotated such that the coil housing and housing adapter move along longitudinal axis 14 relative to metallized diaphragm 52. Since electrode 35 is rigidly attached to the housing adapter, the electrode will move relative to the diaphragm thereby causing the capacitance between the electrode and the diaphragm to vary.

It is desirable to attenuate the very low frequency oscillations of the heartbeat so that a better signal to noise ratio may be obtainedfor vibrations above the frequency of the beat of the heart, that is, frequencies above about one cycle per second. This is accomplished by a filter which filters out frequencies below about 1 c.p.s. This filter comprises a membrane 57 disposed over the diaphragm 52 and spaced from the diaphragm by clamping ring 53 forming a pocket (sometimes called an air space) therebetween. This membrane may be a Mylar sheet about 0.00025" thick. A second clamping ring 58 is used to attach membrane 57 to the body 12 with screws such as 59 shown in FIG. 3. A suitable spacing between the diaphragm and the membrane is approximately of an inch. A hole 60 extends through the clamping ring 53 and the body 12 to allow air to pass from the pocket to the atmosphere. The hole is approximately five to ten thousandths of an inch in diameter such that the time required to evacuate the pocket of air is about one second.

The microphone acts as a sonic detector whereby vibrations from substantially zero c.p.s. to frequencies high in the sonic spectrum will strike membrane 57 causing the membrane to vibrate accordingly. Pulsations having a frequency from zero to one cycle per second have a relatively long period of oscillation in comparison to the higher frequencies. Upon striking the outer membrane 57, these low frequency impulses will cause air to pass from the pocket between membrane 57 and diaphragm 52 through opening 60 as the membrane vibrates transferring relatively little vibration to the diaphragm 52. However, at frequencies above one c.p.s. do not cause appreciable air fiow through the hole 60, due to the inertia of vibrations detected by the microphone.

the air. These will be transmitted to the diaphragm 52 causing the diaphragm to oscillate accordingly. The lower frequencies will thereby be mechanically filtered out. These lower frequencies include those derived from slow movements of the chest due to breathing or shifting of the body, as well as certain low-frequency movements within the heart itself.

The electric capacitance between electrode 35 and the metallized diaphragm 52 is determined by the spacing between the diaphragm and the electrode and a vibration of the metallized diaphragm 52 along the longitudinal axis will cause the capacitance to vary accordingly. The transformer has a tuned secondary comprising the secondary winding 20 t0 the transformer, the fixed capacitor 34, and the variable capacitor as defined by electrode 35 and metallized diaphragm 52 all connected in parallel. The reactance of the capacitors in the tuned secondary is reflected to the primary coil 19 of the transformer and as the spacing between the diaphragm 52 and electrode 35 changes, the effective reactance looking at the primary coil will vary accordingly.

A particular application of this invention is to detect sound waves of frequencies from about one c.p.s. to about 5000 c.p.s. emanating from the heart. The microphone is connected through a shielded coaxial cable to an RF oscillator and the output of the oscillator is connected to an oscilloscope to provide a visual presentation of the The shield of the cable electrically connects the body and coil housing to ground potential. In use, the membrane 57 of the microphone is coated with petroleum jelly and the microphone is applied to the chest wall of the patient so that the membrane is closely coupled to the chest. Optimum results are obtained when the petroleum jelly provides good surface contact between membrane 57 of the microphone and the surface of theskin with no air therebetween. Any movements of the chest wall will be transmitted directly to the membrane of the microphone.

Locking screw 41 is loosened and the microphone sensitivity is adjusted by rotating adjusting barrel 44 until the spacing between metallized diaphragm 52 and electrode 35 produces the optimum signal to noise ratio where the desired information is best presented on the oscilloscope. Because of the keyways 43a and guide screw 42, the electrode will not rotate about the longitudinal axis of the transducer. This will minimize changes in capacitance caused by any irregular surface configuration in either the electrode or the diaphragm. Locking screw 41 is tightened after the sensitivity is adjusted to maintain a consistent spacing between the diaphragm and the electrode when using the microphone.

The microphone output at RF connector 15 may be connected to a detecting device such as an RF oscillator where the primary coil is an integral element of the tuned circuit of the oscillator. A change in the spacing between the diaphragm and the electrode will cause a change in the frequency of oscillation of the RF oscillator where the output signal strength of the oscillator is proportional to the frequency of oscillation. The output of the oscillator may be connected to the vertical amplifiers of the oscilloscope in which the horizontal sweep circuit is triggered by the heartbeat taken from voltages induced from an electrocardiograph of the patient. This will put a presentation upon the screen of the oscilloscope displaying the frequencies detected by the microphone as a function of the output of the oscillator for each heartbeat.

Other means for measuring the output of the microphone are useful in addition to that described, which other means include, for example, the use of DC. polarized voltage, and a capacitance bridge. a

This invention is not to be limited by the embodiment shown in thedrawings and described in the description which is given by way of example and not limitation, but only in accordance with the scope of the appended claims.

What is claimed is: I

1. A microphone comprising a capacitor and a filter, said capacitor having a first plate and a second plate, said plates being spaced from each other and having an electrical capacitance between them which varies according to the spacing between said plates, said spacing between said plates being variable by externally produced sound waves striking said first plate, said filter comprising a membrane spaced from said first plate forming a pocket therebetween, the pocket being closed, therebeing a restricted fluid passageway from the pocket to atmosphere so proportioned as to permit flow of air therethrough in response to forces of relatively low frequencies imposed on the membrane, and to impede said flow in response to forces of relatively higher frequencies imposed on the membrane, whereby to act as a mechanical filter limiting the transmission of the lower frequencies to the first plate. 2. A microphone in accordance with claim 1 wherein said spacing between said first plate and said second plate is adjustable.

3. A microphone in accordance with claim 1 wherein said capacitor is electrically connected as part of a resonant circuit, said resonant circuit having a resonant frequency of oscillation, said variation of said capacitance causing said resonant frequency of oscillation to vary.

4. A microphone comprising a capacitor and a coil and a filter, said capacitor and said coil being electrically connected in a resonant circuit havinga resonant frequency of oscillation, said capacitor comprising a first plate attached to a housing and a second plate disposed within the housing and spaced from said first plate, said capacitor plates having an electrical capacitance which varies according to the spacing between said plates, said filter comprising a membrane disposed on the opposite side of said first plate from said second plate, said membrane spaced from said first plate and forming a pocket therebetween, said pocket containing a fluid, an opening extending from said pocket to the atmosphere to allow said fluid to pass in and out of said pocket, the pocket being closed, there being a restricted fluid passageway from the pocket to atmosphere so proportioned as to permit flow of air therethrough in response to forces of relatively low frequencies imposed on the membrane, and to impede said flow in response to forces of relatively higher frequencies imposed on the membrane, whereby to act as a mechanical filter limiting the transmission of the lower frequencies to the first plate, said membrane being caused to vibrate by externally produced compression Waves, said spacing between said plates being 'variable by compression waves striking said first plate from said fluid.

5. A microphone in accordance with claim 4 wherein said capacitor is connected in parallel with said coil.

6. A microphone in accordance with claim 4 wherein said spacing between said first plate and said second plate is adjustable.

7. A microphone in accordance with claim 4 wherein said resonant circuit comprises a second capacitor, said second capacitor acting to stabilize said resonant frequency of oscillation with changes in temperature of said microphone.

8. A microphone in accordance with claim 7 wherein said second capacitor is connected in parallel with said coil.

9. A microphone comprising a capacitor and a coil and a filter, said capacitor and said coil being electrically connected in a resonant circuit having a resonant frequencyof oscillation, said resonant circuit comprising a second capacitor, said second capacitor acting to maintain impedance stability of said resonant circuit with changes in temperature of said microphone, said capacitor comprising a first plate attached to a housing and a second plate disposed within said housing and spaced from said first plate, said capacitor plates having an electrical capacitance which varies according to the spacing between said plates, said spacing between said plates being variable by vibrations from sound waves, said coil having a primary winding and a secondary winding mutually inductive with said primary winding, said capacitor being electrically connected to said secondary winding, said spacing between said first plate and said second plate being adjustable, said filter blocking undesirable vibrations from varying said spacing between said plates.

10. A microphone comprising a capacitor and a filter, said capacitor comprising a first plate and a second plate, said plates being spaced from each other and having an electrical capacitance which varies according to the spac-- ing between said plates, said filter comprising a membrane spaced from the first plate forming a pocket therebetween and disposed on the side opposite the second plate, said membrane being adapted to be closely coupled to a body which has an outer surface subject to externally caused changes in its contour, said membrane being caused to vibrate by compression waves originating within said body, the shape and volume of said pocket being changeable in response to said compression waves and changes in contour, said spacing between said plates being variable by compression waves striking said first plate, the pocket being closed, there being a restricted fluid passageway from the pocket to atmosphere so proportioned as to permit fiow of air therethrough in response to forces of relatively low frequencies imposed on the membrane, and to impede said flow in response to forces of relatively higher frequencies imposed on the membrane, whereby to act as a mechanical filter restricting the transmission of lower frequencies to the first plate.

11. A microphone in accordance with claim 10 wherein said spacing between said first plate and said second plate is adjustable.

12. A microphone in accordance with claim 10 wherein said first plate comprises a diaphragm and a metallized layer disposed adjacent one side thereof, said diaphragm being an electrical insulator, said metallized layer disposed on the side of said diaphragm opposite said second plate.

13. A microphone in accordance with claim 10 wherein said capacitor is electrically connected as part of a resonant circuit, said resonant circuit having a resonant frequency of oscillation, said variation of said capacitance causing said resonant frequency of oscillation to vary.

References {liter by the Examiner UNITED STATES PATENTS 7 1,456,538 5/1923 Crandall l79-l06 1,760,896 6/1930 Childowsky et a1. 179l06 2,683,861 7/1954 Vierling et al 179l06 X FOREIGN PATENTS 133,527 10/1919 Great Britain.

KATHLEEN H. CLAFFY, Primary Examiner. R. MURRAY, Assistant Examiner. 

1. A MICROPHONE COMPRISING A CAPACITOR AND A FILTER, SAID CAPACITOR HAVING A FIRST PLATE AND A SECOND PLATE, SAID PLATES BEING SPACED FROM EACH OTHER AND HAVING AN ELECTRICAL CAPACITANCE BETWEEN THEM WHICH VARIES ACCORDING TO THE SPACING BETWEEN SAID PLATES, SAID SPACING BETWEEN SAID PLATES BEING VARIABLE BY EXTERNALLY PRODUCED SOUND WAVES STRIKING SAID FIRST PLATE, SAID FILTER COMPRISING A MEMBRANE SPACED FROM SAID FIRST PLATE FORMING A POCKET THEREBETWEEN, THE POCKET BEING CLOSED, THERE BEING A RESTRICTED FLUID PASSAGEWAY FROM THE POCKET TO ATMOSPHERE SO PROPORTIONED AS TO PERMIT FLOW OF AIR THERETHROUGH IN RESPONSE TO FORCES OF RELATIVELY LOW FREQUENCIES IMPOSED ON THE MEMBRANE, AND TO IMPEDE SAID FLOW IN RESPONSE TO 